Blackwood

Except in certain situations where 4NT is a natural bid, it is
an artificial bid initiating Blackwood. In the most basic original form
of Blackwood, the partner of 4NT bidder responds with 5C if he holds
zero or four aces, 5D for one ace, 5H for two, and 5S for three. The
5C bid is not ambiguous because it is generally impossible for a
partnership to be considering slam if they lack all four aces, and in
the weird exceptions, 4NT bidder knows whether to expect a strong hand
(that could have all 4 aces) with pa rtner or not.

In any case, the 4NT bidder may continue Blackwood by bidding 5NT,
which guarantees he holds all the aces or key cards that 4NT was
asking for, and asks for kings with the same scheme. If the king
of trump was counted in the 4NT bidding, it doesn't count again;
usually the queen of trump counts as a king in its place.

Gerber is a similar convention used to ask for aces on a no-trump
hand.